There are numbers of all kinds: rational, real, complex, p-adic.
The p-adic numbers are less well known than the others, but they play a fundamental role in number theory and in other parts of mathematics.
This elementary introduction offers a broad understanding of p-adic numbers.
From the reviews: "It is perhaps the most suitable text for beginners, and I shall definitely recommend it to anyone who asks me what a p-adic number is." --THE MATHEMATICAL GAZETTE